Audiometer



AUDIOMETER Filed-Aug. 1., 1940 6 Sheets-Sheet 1 R O T N E V m Mmmm Skdkmlm ATTOR NEY Sept. 30, 1941..V H, wfKoREN-a AUDIOMETER Filed Aug. l, 1940 2 A INVENTOR Hma/n wm w8.. @za ////ll l l l l l ATTORNEYS sept. 3o, 1941. Hw, KQREN l AUDIO'METER vFiled Aug. l, 1940 G Sheets-Sheet 4 rhlllllHH IIHIH Illlllllllllll 7 .y MM? am um a .M ,g

ATTORNEY sept. 3o, 1941. H w KQR 2,257,263

' AUDIOMETER v Filed Aug. l, 1940 Sheers-Sheet 6 ZIO ' ATTORNEY.

,Patented Septr30, 1941 f lum'rfribf,s'lrli'riezjsl PATENT `oFFl-cs ,l y autres: j'

naman w. xosrm; Y., u

Sonotone Corporati 011, ElmsfonLvN. Y., a oorporation of New York This application is a continuation-impart of my application Serial No. 277,882, filed June 7,

This invention relates to measuring arrangements, such as audiometers. Among the objects of the invention` is a novel combination of an electric oscillation generator arrangement and a sound reproducer supplied thereby with an attenuator arrangement for accurately regulating the sound output at different frequencies for measuring the hearing sensitivity of an individual, and similar applications; and improved methods and arrangements for controlling and operating such equipments.

I'he foregoing and other objects of. the invention will be best understood from the following description of exemplications thereof, reference being had to the accompanying drawings wherein Fig. 1 is a diagram showing the circuit connections and the cooperative relationship ofthe elements of an audiometer utilizing an oscillation generator and exemplifying certain other fea- Ktures of the invention;

Fig. 2 is a diagram showing acteristcs of a normal person;

Fig. 3 is an elevational v away illustrating the panel of an audiometer designed in accordance with the principles illustrated diagrammatically` in Fig. 1;

the hearing char- Fig. 4 is a sectional view with parts broken i away. showing .a part of the mechanism of the audiometer of Fig. 2, as illustrated diagrammatlcauy in Fig. 1;

Fig. 5 is a. horizontal sectional view along line Fig. 6 is a sectional view along line 6-6 of Fig. 5;

Fig. 7 is a cross-sectional view along line of Fig. 3;

Fig. 8 is across-,sectional view along line 8-8 of Fig. 7 illustrating the interior of the attentuator mechanism;

Fig. 9 is a plan view of some of the elements of the mechanism shown in Fig. 4; and

Fig. 10 is a diagram similar to'Fig. 1 illustrating another audiometer arrangement exemplifyi'ng the invention. 1

An ideal audiometer should enable a person without technical skill to make accurate measurements of the hearing sensitivity of an indivldual over the principal frequency range of audible sound. The design of a satisfactory audiometer must take into consideration the hearing characteristics of a person having norview with parts broken l y Anuman august 1, 1940, serial No. 349,045

, ,ss claim.

'Iypical results of such measurements are shown' f 5 in the audiogram of Fig. 2 wherein the lower curve marked' Threshold of audibility and the upper curve marked Threshold ofv feeling give the limits of the hearing ability of a person hav-- ing normal hearing. The lower curve shows. for each frequency, the intensity of f the sound in decibels, above an arbitrarily chosen level, at which the tone is barely audible and represents the minimum audibility curve. The' upper curve gives for each frequency the intensity of .the sound which is so loud that it produces a sensation of feeling and becomes painful and represents the maximum audibility curve. The points at which these two curves intersect give the lower and upper frequency limits of, audibility.` These lcurves are useful for making measurements of hearing defects and in computing the percentage of hearingl or the percentage of hearing loss of a-person having defective hearing.

lis shown by the audiogram, the sound intensity giving the lower threshold of hearing, varies unequally in opposite directions while the frequency range is swept from one to the other end, dropping, as the frequency is increasedfrom its lower end, by about 90 decibels to'va minimum, and again rising by about 90 decibels as the frequency is increased to the upper limit of the audible frequency range. The curves marked 20 to 80 give the intensity of the sound at the diierent frequencies which are 20, 40, 60 and 80 decibels,

hearing, each curve. giving the sound intensity which produces' soundV of the same hearing sensation level above the threshold of hearing throughout the audible frequency range. y

As shown in Fig. 2, the useful range of hearing at each frequency may be expressed in terms oi the number of decibels between the threshold of hearing and the threshold of feeling. The frequency range which is important for hearing and in which the range of hearing sensitivity is greatest lies between about 100 and 8000 cycles. In the mid portion of this frequency range between about 500 and 2000 cycles the range of hearing sensitivity is a maximum, amounting to about 130 db. above the threshold of hearing. In the lower and higher parts of the frequency spectrum, the range of the hearing sensitivity goes te down, being only about 15o ab. at 256 and 4096 respectively above the lower threshold of normal For determining the hearing deafened person, a satisfactory audiometer should be able to measure 80% of the loss of hearing in the principal audible frequency range between about 100 and 8000 cycles. In addition, a good audiometer should be able to impress at each frequency. within the testing range,y a substantially pure tone, and the intensity of the tone should be accurately measurable. A change in sound intensity by one decibel is just about the minimum change detectable by the human ear. However, on account of the uncertainty of judgment of an average individual, one decibel change in sound intensity is lof no great significance and an audiometer able to make measurements of the hearing sensitivity within about plus or minus 3 decibels is satisfactory.

Although audiometers capable of producing pure tones in frequency steps that are an octave apart in the range between about 100 and 8000 cycles meet many needs, there are many conditions requiring audiometers with a continuously variable frequency range. For instance, audiometers having a continuously variable frequency make it'possible to detect peculiarities in the hearing of an individual 'at points which lie be-v tween the fixed frequencies which audiometers With fixed frequency steps would not detect.

Fig. 1 is a diagrammatic illustration of a novel y ypairmernt of av ,l `terr-:aac cycles. and only about 90 db. at 128 and 8192 from an A. C. or D. C. 11o-volt domestic supply line, indicated by the plug 32, a `filtered positive plate -or anode voltage at the terminal +33 to` the power amplier stage, and at the terminal +34 to the preceding amplifier stages. The cathf. odes of the several tube units are indirectly audiometer exemplifying one practical embodiment of the invention. Although an oscillator arrangement of the invention ofthe type described in connection with Fig. 1 using only a single feed back network with resistors and variable condensers is sufficient for producing cillations continuously variable over the entire audiobfrequency range important for practical audiometers, there are various applications in which it is desirable to extend the range of the frequency obtainable with a single feed back network lof such character and accordingly, the audiometer arrangement is shown provided with two feed back networks which are automatically switched into the oscillation amplifier circuit as the frequency is varied from one part of the frequency range to the other.

It 4comprises an oscillation generator utilizing the amplifier tubes 2|, 22, 24, 26, 28 for generating the oscillations which are supplied through an attenuatorl and the associated circuits to a receiver through which sound is impressed on the hearing organs. The oscillation generator` shown4 is designed in accordance with the principles disclosed and claimed in my copending application Serial No. 395,927, filed May 31, 1941, as a continuation-in-part ofthe present application.

The oscillations are produced by electron amplier tube units 2|, 22` and a feed back network LF or 23 interconnected between their input and output sides, and the generated oscillations are impressed by the two push-pull connected ampiifier tube units 22,24 on the two push-pull connected amplifier tubeunits 26, 21 forming the power amplifier stage which delivers the oscillations to an output transformer 29, the secondary winding of which is connected to a load.

In the form shown, the amplifier is designed to be energized from a conventional power supply system comprising, for instance, a rectifier 3| and a filter network formed of chokecoils, resistors and condensers interconnected so as to deliver heated by heater filaments not shown, which are likewise supplied through plug 32.

The first oscillation amplifier tube unit 2| op-` erates as a high gain amplifler and is shown in the form of a pentode having its anode connected through a coupling resistance 4| and a current adjusting resistor 42 to the positive supply terminal +34, its screen grid being maintained at the required positive operating potential through a resistance 43, interconnected between the resistances 4|, 42 and its'cathode being connected through a self-biasing resistor 44 to the low potential or grounded side of the amplifier indicated by a lead G, suitably connected by-pass' condensers completing the circuits.

lThe Voutput of they first amplifier tube is impressed through a blocking condenser 46 on the input grid of tube unit 22 which is connected through a grid resistor 41 to ground. The tube unit 22 operates as a phase inverter and is shown in the form of a triode which impresses on its output coupling resistor 48 a voltage which is- Ain phase with the yoltage impressed on the ing put grid of the first amplifier .tube 2|. l

'Ihe voltage developed across the coupling resistance 48 of the triode 22 is impressed through a blocking condenser 5| and a feed back lead l52 including a serially connected resistance 53 upon the entrance terminals of either feed back network LF or the feed back network HF, the outgoing terminals of which are connected between the input grid and the low potential or' grounded side of the first amplifier tube 2|.

Each feed back network is formed of av first network section consisting of a series resistance RI and a shunt condenser C| and a second network section consisting of a series condenser C2 and a shunt condenser R2.

An adjustable-tap resistor 54 is connected across the entrance terminals of each feed backv network 23 to form a voltage divider, enabling accurate adjustment of the component of the output voltage impressed on the entrance terminals of the feed back network.

Sustained oscillations will be generated by such arrangement at the frequency Fo given by:

1 F 2xt/alamo, (l) In such arrangement, with the first tube 2| operating as an oscillation amplifier which shifts the input voltage 180, and the second tube unit 22 as a phase inverter having an output voltage ,network will produce only at the frequency Fn given by Equation 1 an overall phase shift zero, and feed back to the input grid of the first tube 2| a voltage which is in phase with its grid input voltage of tube 2 and bring about the generation of oscillations of the desired frequency if the amplifier has an overall gain sufiicient to cover the losses in the feed back network 23, and in thev other` circuit elements associated with the oscillating circuit.

The condensers C4 are connected in shunt to the feed back resistors RI, and are effective in increasing the range over which the frequency of the oscillations may be continuously varied by the aforesaid application.-y l

A 4practical oscillator actually constructed'in the way indicated in Fig. lwitlr a feed back network using variable condenser Cl, C2 having a maximum capacity of '130x 10-u farad and rev quirlng three different sets of feed back resistors interchangeably switched into the feed back circuits for covering in three steps the ranges from l2 to 200 cycles, from 200 to 2,000 cycles, and from 2,000 to 20,000 cycles, on being modified in accordance with the invention by combining with it a negativelyacting condenser C4 of 100 10u farad connected in shunt to the feed back resistor RI, and using the same set of resistors RI, R2,

enables an increase of the frequency range over which the frequency was continuously 'variableV by 100 times with only a single set of such feed back resistors.

In the form of the oscillator arrangement shown in Figf 1, the triode 22 which serves as a phase invertor element in the oscillation generating circuit is utilizedJ also to cooperate with the associated triode v24 as a push-pull exciting stage Vfor driving the push-pull amplifier stage with its power'amplier tubes 20, 28. 'The cathodes of the two triodes 22, 2l are connected in A their push-pull circuit through balancing selfbiasing resistors, and their two anodes are connected `in the push-pull circuit across two coup'ling resistors Il, the high potential points of which are connected through similar blocking condensers 5| to the high potential points of the push-pull connected grid resistors Si of the power amplifier tubes 26, 28, the cathodes of whichare likewise' connected in their push-pull circuit.

way by automatically controlling its gain so as to operate the oscillation amplifier as a class A ampliiier, restricting the oscillations to the linear part of the tube characteristics and by applying a suitable amount of negative feed back.

In the arrangement of Fig. 1, the self-biasing -resistor 44 has no shunting by-pass condenser and is connected in series with the cathode of the oscillation amplifier tube 2|y in such relation to the. resistor 4| .of its'ano'de circuit as to apply to its grid a negative feed back'voltage sumcient' to reduce distortion, but insufficient to stop oscillations. The cathode resistor I4 maybe used to automatically control the gain of the ampli iler tube by making it in the formo! an incandescent 'lamp filament arranged to operate at a temperature at whichits resistance rises with the current so as to automatically vary the grid bias of the amplifier tube 2| and maintain its operation in the linear part of its characteristic at a gain which is just sumcient to supply the losses in the feed back network. and the other parts of the oscillator circuit and produce the desired sustained oscillations. A

In the audiometer of Fig. 1, the amplier tube units 2|, 22, 24, 28, 28 are connected toproduce oscillations when either the feed back network 75 t f ananas l l mandenme ci, cz with angle et or feed back resistors in the way disclosed in my u' r the feed 'tack network Hr ls contestes by the two relays Il, 82 `between the output side of the'aniplifier triode 22 and the input side of the amplifier tube 2|.

5 The parameters of the feed back network LF are chosen to produce oscillations variable over a lower part of the 'frequency range and the parameters of the feed back network HF are f chosen to produce oscillations variable over: a 14') higher part of the frequency range. The component of the feed back voltage applied to the` input side of each feed back network is adjusted by a variable tap resistor Il and linear operationv of the oscillation generating amplifier over 15 the frequency range is assured by the automatic control of the bias of the grid of the amplifier` tube 2l through a rectifier arrangement including` a rectiner 88.

The relays 8|, 82 are of the direct current typeand have their coils connected in series between ground and a positive terminal which is connected through a filter network including an inductance, a current limiting resistor, anda condenser tothe rectifier 8| of the power supply 25 system. When the relay 8| is energized, as in the position shown in Fig.`1, its two s ets of downwardly flexed contacts connect the input grid of the amplifier tube 2| and the lead from the adjustable tap of the rectifier resistor 64 to the terminals of the resistor R2 forming the outgoing terminals of the feed back network LF, the

corresponding terminals oi the other feed back` network HF being connectedto the tube circuits in the released normally biased position of the contacts of relay 8|. i

When the other relay 82 is deenergized, as in the condition shown in Fig. 1, its upper released contacts complete the feed back connection between resistor 53 and the entrance terminal of 40 the feed back resistor Ri of the feed back network LF, the corresponding terminal of the other feed back network HF beingsimilarly interconnected when the relay 82 is energized.

In the position shown in Fig. -1, the relay 82 is held deenergized -by completing at the closed contacts of an auxiliary switch 83 a short circuit around its actuating coil, the switch 83 being so arranged that when it opens and restores the energization of the relay 82, the latter completes at its lower exible contactsa short circuit for the coil of the other relay 8|, thereby releasing its two sets of contacts to their normally biased unexed position. Y

In other words, when the auxiliary switch 83 is closed and completes its circuit, the energizing relay 8| and the deenergizing relay 82 connect the feed back network LF in the oscillator circuit, and when the switch 83 is opened, thereby energizing relay 82 `and deenergizing relay 8 I, the

so two relays disconnect the feed back network LF and connect instead the feed back Vnetwork HF in the oscillator circuit.

The frequency of the generated oscillations is `continuously varied over a large continuously variable frequency range made possible by the n use of several of such phase-shifting or control` ling networks LF, HF by a frequency selector or control arrangement 80 which is so designed that by a continuous movement of a single frequency selector such as member 9|, the generated oscillations are continuously fvaried from one end `of the desired frequency range to its other end.

the frequency selector 8| has a driving gear 82 In the form shown diagrammatically in Fig. 1,

which, in theposition shown, drives a condenser gear 93 mounted on a common shaft 94 with the rotors of the variable condensers C| C2 of the feed back network LF, so that movement of the frequency selector 9| to the left actuates the condensers CI, C2 to continuously increase their capacity thereby raising the frequency of the gen-4 lar condenser driving gear 95 which rotates through a common shaft 94 the variable condensers CI, C2 of the feed back network HF so as to vary them ini a similar way in the course of the further motion of the frequency selector 9| while the right end of its gear 92 leaves its driving engagement with the condenser driving gear 93 the feed back network LF.

'I'he frequency selector 9| is also provided with two condenser locking cam sections ,96,'91'1V adjoining the ends of its driving Agear 92 and ar' ranged so that locking cam section 96 maintains locking engagement with a locking projection 98 of condenser gear 95 of the network HF in order to keep its variable condenser C. C. locked in the same operating position as long as the selector gear 92 rotates the variable condensers .C|, IC2 of the other network LF and that the frequency selector gear` 92 in the course of its leftward motion reaches the position in which it disengages from the condenser gear 93 of the network LF and engages the condenser gear 95 of theother network HF the locking projection 98 of its condenser gear 95 is released by locking cam section 96 of the frequency 'selector 9|, and its other locking cam section 91 establishes a similar locking engagement with a similar locking projection 98 of the condenser gear 93 of the network LF in order to keep its l variable condensers- CI, C2

.locked in the same operating positions as long as selector gear 92 rotates the variable condensers Cl, C2 of the feed back network HF.

In`other words, the frequency selector 9| is so the relays 8|,l 82 yclosed as long as the condenser gear 93 is being rotated, and to open the contacts of the switch 83 only at the point when, during the transition period, the contacts of the switch 83 are brought into engagement with an interrupter member 99 `operated by condenser gear 93.

In the practical construction of such control arrangement, the condenser gear 93 is made in the form of a. metal disc, of brass, for instance,

and thel contacts of the switch 83 are biased toy ,clockwise rotation starts to enter between the contacts of the control/switch 83 and opens the switch circuit thereby actuating the relays 8|, 82 to disconnect the feed back network LF and connect the feed back network HF into the oscil- 4 lator circuit.

vThis switching occurs at the moment of the transition, period while the frequency selector gear 92 is in driving engagement with condenser gears 93, 95 of both feed back networks LF. HF,

the driving engagementlof the frequency selector 9 with the condenser gear 93 being broken when the points of the switch contacts 83 reach an intermediate portion of the insulating interrupter insert 99, this being the position when` the condenser locking section 91 establishes its locking engagement with the locking projection 98 of condenser gear 93, as indicated by their dotted line positions 98', 99', respectively.

i In other Words, in the frequency selector ar-` rangement described above, the frequency selector 9| with its various elements are so arranged that as the frequency selector "9| is moved from the position shown toward the left, it will continue to rotate the condenser gear 93 of the frequency selector LF until, at the .point when its condensers Cl, C2 are about to reach their position L, of minimum capacity, the driving gear 92 of the frequency selector also establishes driving enarranged that during a short transition portion of its motion when it breaks the driving connection to the variable condensers of one feed back network and establishes its driving connection with the variable condensers of the adjacent feed back network, it remains in driving engagement with the variable condensers of both feed back networks, thus providing a period of transition dur- Iing which the frequency controlling elements of both feed back networks are varied, the feed back networks being so designed that their ranges overlap and merge during the period of transition. v

Provision is also made to assure that during the transition period of the frequency selector 9| as it leaves its coupling engagement with the control element-of one feed back network and establishes its coupling engagement with the con'- trol element of the succeeding feed back network, the switching relays 8|, 82 are automatically actuated to disconnect one feed back network from the oscillator circuit at the moment during when the two feed back networks are in a condition at which they generate oscillations of the same frequency.

gagement with the condenser gear 95 of the feed back network HF; and 'that during the transitionl period ofthe further movement of the frequency selector 9| toward the left, while -both condenser gears 93, 95 are rotated, a point is reached at which the relay control switch 93 is opened at the moment when both feed back In the form shown, such automatic action is V obtained by utilizing `the condenser gear 93 Vto keep the contacts ofthe control switch 83 of networks are in a condition in which they will generate the same frequency, the switching operation of the relays connecting at that moment the feed back network HF in lieu of feed back network LF into the oscillator circuit; and that when, at the end of the transition period of the motion of the frequency selector 9| its gear. releases the condenser gear 93 of the network LF and locks it by its locking cam 91, the insulating interrupter insert 99 of the gear 93 will hold the relay control switch 83 open and keep the feed. back network HF connected in the oscillator circuit, as long as the frequency selector 9| rotates the condensers CI, C2 of the network HF for varying the frequency over the other range; and that `on the reversal of/the movement of the frequency selector 9| from its utmostleft-han'd position, asimilar sequence of operations is performed in reverse order.

, Simple continuous variable control of the generated frequency, as indicated by the frequency aired. 'Thus in the specinc exemplification of the index on the frequency scale over a larger range than possible with a single rfeed back-network, is thus made possible by avery simple and foolproof mechanism.

In order to assure that the frequency generated by` the two feed back networks LF, H1'I at the moment of transition is identical, small trimming condensers or trimming resistors may combine with the feed back networks. I'he moment at which the relay control switch 03 is actuated from the closed to the open position for cutting out one feed back network and interconnecting the other into the oscillatorl circuit may be adjusted by an'adjustable mounting of the switch 00, so that it may be shifted'in the direction indicated by the arrow and locked in the adjusted position, corresponding to the position when the frequency generated by the two feed back networks are identical. The adjustable resistors 54 connected to the entrance terminals of the two feed back networks make it possible to adjust their operation to assure that at the moment of transition from one feed back network to the other, not only the frequency of the generated oscillations shall be identical, but that the oscillations impressed on the output transformer shall be of the same intensity.

` By an arrangement of the type described above, a simple motion imparted to the frequency selector 9|, from the low frequency end of its scale to the high frequency end of its scale as indicated on its index will actuate the oscillator circuits to generate the oscillations varying over thelarge frequency range made possible by the use of two or more feed back networks covering successive parts of the desired frequency spectrum which are automatically switched into the oscillating circuit asthe frequency is varied.

The oscillations of the different frequencies are delivered to an attenuator associated with attenuation dial 2|.

Attenuators for making audiometric measurements have to be so designed as to enable attenuation of the electric output delivered to the receiver in terms of a logarithmicscale expressed as the hearing loss in decibels above the lower threshold of hearing of a normal person.

In the form shown, the attenuator |00 comprises a wire-wound resistor pad |0| arranged to be engaged by a relatively movable slider contact |02 in the way'generally used in the known volume controls, and an adjacently mounted second wire-wound resistor pad |05 and a bus conductor |04. A plurality of spaced points along the resistor pad |0| are electrically connected by jumpers |03 to a plurality of equally spaced points on the resistor pad |05 andv the mid Points of `the consecutive vresistor sections of the resistor pad |05 are interconnected by jumpers |06 to the bus conductor '|04. If the resistor pads |0I, |00 are uniformly wound, the consecutive resistor sections of the rheostat pad |0| represent the line resistance of a logarithmic attenuator and two adjacent resistor sections of the resistor |03,

which connect each jumper |05 to the bus conductor |04, act as leak resistors and represent'the elements of the distributed leakage resistance along the line. Two wire resistor p ads, interconnected by jumpers and having a slider contact on one pad will give as accurate attenuation as the cumbersome prior art wire-wound attenuators. and the error occurring when the slider contact |02 engages a point of the resistor pad |00 between adjacent jumper taps can be kept down to as small a fraction of adecibel as de- |00 of 4600 ohms, and interconnecting the resistance pad |0| at twelve equally spaced points with the resistor pad |03 and connecting 24 equally spaced points of the resistor pad |00 to the bus conductor |04, and continuous logarithmically uniform attenuation with an error much smaller than 1 db. is secured by moving the sliding contact |02 along the rheostat pad |0 i.

The attenuator scales ABA, ABB. for air conduction or bone conductionI are arranged to indicate, -for each attenuator setting, the intensity of the sound produced by the receiver AR, BR respectively in terms of decibels hearing loss above the intensity of sound which produces a barely audible sound, i. e., above the soundintensity of the lower threshold of hearing as indicated in the lower,curve of Fig. 2 which indicates, for each frequency, the point of zero sensation level for the air-conduction attenuation scale ASA. In the form shown, the slider contact on |02 is ailixed to the attenuator scale AS and the attenuation index AI is in a fixed position so as to indicate on the attenuation scale the attenuation for the different positions of the slider contact |02 relatively to the resistor pad |0| of the attenuator.

In the arrangement shown, the input side of the attenuator network is connected through leads |01 tothe secondary winding of the output transformer 29 of the oscillator, a terminal resistor |00 connected across the input terminals 0f the attenuator serving to match the load impedance of the attenuator network with the output circuit of the amplier.

TheA bus conductor |04 and the rod or lead |09 on which the slider contact |02 of the attenuator rides deliver the attenuated output, the degree of attenuation being indicated by an attenuator index shown in the form of an arrow marked AI- on an attenuator scale AS. The audiometer shown in Fig. 1 is intended for operation either with an air conduction receiver AR blade, the attenuator output leads I 04, |09, by

way of -an impedance matching transformer ||4 and a serially connected condenser to the airconduction receiver AR, the bone conduction receiver BR being connected directly to the attenuator output leads |04, I 09 in the other position of the switch blade. In order to prevent the flow of capacity currents from the attenuator network to the receiver, one side of the attenuator, namely the connectionof its bus lead to a low potential end of the secondary winding of the output transformer 291s connected to ground through the middle blade 4of the changeover switch"||0.

When the bone conduction receiver BR is'directly connected to the attenuator output leads |04, |09, a direct connection of oneside of the receiver to ground would result in the flow of capacitive currents from the primary to the secondary winding of the output transformer. This is avoided by using the middle blade of the changeoverswitch ||0-to disconnect the ground from the secondary transformer winding and establishthroi-igh a small condenser I I6 of about '.02 mlcrofarad, for instance, the required byrelaxation oscillator formed of a gaseous conduction tube III and the associated circuits is receiver MR, which supplies the masking noise.'

used as a source of noise currents which are impressed through an output transformer and an auxiliary attenuator H8 on an air conduction son in the principal audible frequency range between about 100 and 8000 cycles, this means that the minimum output of the receiver must be The cathode of the oscillation generator tube lH9 is supplied from the power supply plugs 82 and its plate circuit is energized from a positive lead of the power system rectifier 8| by way of a circuit whichis completed by a jumper extending between plug terminal contacts of a plug member I I9 which is usedv to connect the masking receiver MR., to the attenuator H8, so that only when the masking receiver is plugged ink by its plug I t9 will the relaxation oscillator Il@ be set into operation to supply noise currents to the masking receiver MR..

' Since the effectiveness of an audiometer as a measuring instrument depends'on the accurate determination of the intensity of the acoustic output delivered by the receiver to the hear-ingergan `of the individual under test, it is essential that the attenuation of the acoustic output of the receiver ls actually varied in accordance with the indications on the attenuator scale. To meet the foregoing requirement, the best audiometers ,available prior to the invention had to resort to complicated attenuation scale arrangements because, as shown in Fig. 2, the sound intensity giving the lower threshold of hearing, and representing the zero hearing level on the attenuatio scale, 'varies unequally in opposite directions while the frequency is varied from one end oi' its scale to its other end, dropping by about 90 decibels to a minimum. as the frequency is increased from the lower limit of the audible frequency range to about' 2000 cycles per second, and rising again by about 90 decibels above the minimum as the frequency is further increased to the upper limit of the audible frequency range.

' Furthermore, the emciency or sensitivity of the available receivers differs greatly over the 'frequency range of audible' hearing, being usually a maximum at about a thousand cycles, and fally ing oil` at lower and higher frequencies. In addition, itis also necessary to compensatev for any variations in the oscillator output at different l f frequencies.

In making'audiometric measurements, it :is also --cesirab1e to be able to sweep throuehthe entire "'uaudible frequency rangewhile impressing at all ,frequencies the sound intensity producing the same sensation level on the tested ear organ. The available receivers which are used for audio- 'ifmetric measurements, while able to produce' the sound intensity corresponding to the highest senr'sation levels indicated on the audiometric scale in the rangebetween 500 and 2500 cycles, are not e `able to produce such sound intensity in the low and high audiofrequency ranges. 'I'his is due to e vthe fact that even the best available air conduc- ,tionalreceivers have very little sensitivity in the lowfrequency range below about 500 cycles and have alsojsubstantially reduced sensitivity in the high frequency range above about 4000 cycles.

Since a satisfactory audiometer should be able tomeasure 80% of the'hearing loss of a Per1 f 400 cycles. However, the best available air conabout 100 db. above the lower threshold of normal hearing in the range of about 1000 cycles where the 'hearing range is a maximum and that the attenuator should be able to indicate such loss. If an audiometer would be designed to deliver a receiver output of 100 db. above the threshold of hearing throughout the frequency range between about 100 cycles to 8000 cycles, it would require not only an oscillation generator of abnormally large output capacity, but also abnormally powerful receivers able to deliver such output in the low frequency range between 100 and duction receivers could not be made to operate satisfactorily with such high power output; and even if they were made to operate with Asuch large output vthey could readily get out of adjustment and their response characteristics would undergo changes each time they were subjected to such abnormal load conditions.

Accordingly, a satisfactory audiometer should enable the testing of the hearing loss up to at least 100 db. in the range between of about 500 and 2500 cycles, and at the same time, it must render it impossible to make on the .attenuator a reading corresponding to a sound intensity which the receiver is unable to deliver at the given frequency setting, such as 125 cycles, at which commercially practical receivers are able to deliver only about db. above the threshold of normal hearing. In other words, an attenuator designed to measure db. hearing loss at 1000 cycles, should render it impossible to make inaccurate measurement of the hearing loss in the frequency ranges, such as at cycles at which the hearing sensitivity of the normal ear is much poorer and the maximum loss that has to be measured, namely about 'l5 db., does not exceed theoutput which commercial receiver is able to deliver at such frequency. All prior i commercially available audiometers-resorted to dials having several scales in order to meet the l most essential requirements, and could not be used for sweeping the frequency range without disturbing the accuracy of the attenuator read-v ings.

v According to the invention, the diiiiculties connected with complicated attenuator scale arrangements of prior audiometers are eliminated, and an audiometer operating with a single attenuation scale and indicator is made possible, by interlinking the oscillator frequency control means and the attenuator control means so as to unequally modify in opposite directions the magnitude of the oscillations supplied by the oscillator to the receiver at the different frequencies and thereby` compensate `for, irregular variations -of the sound energy giving zero hearing level as well as any other irregularities occurring in the operation of the audiometer while the frequency of the oscillations is varied from one end to the other end of the frequency scale.

In the audiometer of Fig. I' 1 embodying one form of the invention, the attenuator is combined in a special way with the frequency selector mechanism so that all the required compensation and the correlation of the settingsof the attenuator to the different settings of the frequency selector are obtained without the use of special compensating resistors, while rendering it possible to sweep through the frequency range settings with an attenuator arrangement which agences f makes it possible to extend. the range or the hearing loss that can be measured with a single scale up to the maximum output which the re. 'ceiver is able to deliver at the frequencies at "tens'ity which the receiver is able to deliver is suilicient to measure the mailer hearing loss in the range of smaller hearing sensitivity.

In the arrangement of the invention, the attenuator is so combined with the frequency selector that if `the frequency selector is swept through the frequency scale, the receiver will deliver the output corresponding to a given attenuation setting as long as. the receiver is able to deliver the required sound intensity; and that as `soon as the frequency selector is moved to a frequency setting at which the receiver is not able to deliver the sound intensity indicated on the attenuation scale, the attenuator is automatically brought to a setting corresponding to 4the maximum sound intensity which the receiver is able to deliver at the given frequencies as indicated on the frequency scale. l

In other words, by the arrangement of the invention. the frequency selector means and the attenuation regulating means are so correlated l that the sound intensity delivered by the receiver v remains at the sensation level indicated on the .attenuation scale as long as the generated frequency is varied throughout the frequency range at which the receiver is able to deliver the sound intensity corresponding to the given. attenuator setting as .indicatedon its scale, while assuring that if thel frequency selector is moved into a part of the frequency range at which the receiver is not able to deliver the sound intensity indicated on the attenuation scale at the given attenuation setting, the attenuator is automatically adjusted to.,give and indicate the correct lower sensation level in the different parts of the frequency range while rendering it impossible to impart to the attenuator asetting correspondlng to a greater sound intensity than the receiver isable todeliver at the given frequency setting to which the frequency selector has been swept or adjusted.

' One form of arrangement for securing such automatic correlation of the frequency selector means and the attenuator arrangement is shown` the slider contact |02, while maintaining at all times engagement therewith.

The attenuator control cam track |2| is engaged by a cam follower pin |25 of a slidably mounted rod |21 which engages through a collar |28 a projection |29 yfrom the attenuator sup.M

port |24, so that when the cam follower rod |21 is moved to different positions by the transverse movement of the cam track |2|,'the attenuator support with its resistor pad is moved relatively to the slider contact |02. The cam fo1 lower rod |21 and the attenuator support |24 are suitably guided so as to keep the attenuator 4in the proper operative position while the cam Y the attenuator support l|24 with the vattenuator pad to different positions of engagement with the slider contact |02 as the frequency selector 5 0| is moved over its frequency range as indicated on. the frequency scale.

In order to maintain the selected attenuatorl setting as indicated on its scale AS by the index byrthe frequency selector cam |20, there is provided a suitable braking means shown in the form of a braking pad |2| pressed by a spring against the attenuator scale AB which is amxed to the attenuator slider contact |02 and moves therewith relatively to the attenuator/index AI.l

This arrangement assures that, normally, `the lmotion imparted by the frequency selector cam |'to the attenuator support |24 does not disturb the attenuator slider contact |02 and the 20 attenuator setting, as indicated on its scale ASB. In order to show in Fig. 1 how the cam tracks |2|, |22 operate to `render it impossible to make Ainaccurate measurements of the hearing loss in the frequency ranges below 600 cycles and above 2500 cycles in which the receiver is not able to deliver the sound intensity required to measure a 100 db. loss. a part of the frequency selector 0| with the associate 4scale and cam surface portions between 800 and 2000 cycles is broken away. The cam follower rod |21 is mounted so that it may be turned as by an arm ill through an angle of 90 so .as to release the follower pin |25 from the cam track |2| and to bring engagement of a similar cam follower pin |2| with the other cam track |22, when tests by a bone conduction receiver are to be made, or vice versa.

The arm |35 of the cam follower rod |21 is also used to actuate the changeover switch ||0 from the position in which the air conduction receiver AR is connected to the attenuator to the position in which the bone conduction receiver BR. is connected to the attenuator, and vice versa, these positions corresponding to the engagement of the cam follower pin |25 with thecam track |2| or cam follower pin I 20 with the cam track |22, respectively.

The cam track |2| for measuring hearing with the air conduction receiver AR is so designed and shaped that for each frequency setting yof the frequency selector 20, as indicated on the frequency selector scale FS, the attenuators support |24 as actuated by the cam follower rod |21 to automatically shift the attenuator pad |0| relatively to the slider contact |02 to a position in which the electric output delivered to the air conduction receiver AR gives a sound output which produces a sensation level of hearing indicated on the attenuator scale ASA, and to automatically assure compensation for. the relative variations in the sensitivity of the receiver and the variations 'of the oscillatory output delivered at each particular frequency.

The cam tracksv 2|, |22 are so designed that fox` each frequency. the attenuation index` AI shall indicate on the attenuation scale AS zero hearing loss corresponding to the lower threshold of `hearing of a normal person, such as indicated by the lower curve in Fig. 5. 'Ihus, if, for instance, the cam track |2|'for the air conduction receiver is to be designed, one may proceed as follows:

The air conduction receiver is applied to the ear of a person having normal hearing in a silent room and the frequency selector is set for one '75 frequency, while the attenuator scale 1s set for track |2| actuates the rod |21 and therethrough.

AI, while the attenuator `support |24 is .movedv nero. Thereupon, the attenuator support |24 is shifted relatively to the slider contact |02 until the normally hearing person signifies that he barely hears the tone. The position of the attenuator support relatively to the cam plate |20 at the particular frequency gives one point of the cam track |2| corresponding to the zero setting l of the attenuator scale at the given frequency.

By proceeding in a. similar way for all other frequencies, a cam track for the entire frequency range is determined. In making such calibration, a number of individuals having good hearing are tested, and the average curve for a number of persons is used as the nal calibration curve for the cam track |2i.

In a similar way, lthe cam track |22 for the bone conduction receiver BR. is calibrated.

In order to show in Fig. l the principal characteristics of the attenuation controlling cam tracks |2|, |22, used in practical audiometers of the invention, the intermediate portion of the frequency scale FS between about 600 cycles and 2500 cycles, in which very little compensation for the variations in the receiver sensitivity and other variables is required, is shown broken away, the broken away cam track portions being substantially even.

In order to prevent making of erroneous readings on the attenuation scales ASA, ASB, provision is made to assure that only in one position of the frequency selector 9| can the audiometer operation be changed from measuring with the air conduction receiver AR on the attenuator scale ASA to measuring with bone conduction receiver AB on` the attenuation scale ASB, and

this is secured by confining one of two guide pins |25', |26', extending from the cam rod |21 in alignment with its corresponding cam follower pins |25, |26, respectively, within a guide channel |29 `so as to permit turning of the cam rod |21 only when the frequency selector 9| is brought to the position of the lowest frequency in which both guide pins |25', |26' of the cam rod |21 are released from the guide channel, and theV cam rod may be turned 90 to bring either its cam pin |25 into engagement with cani track |2| or its cam pin |26' into engagement with cam track |22.

The audiometer arrangement described above assures that at each frequencysetting as indicated on the frequency scale, the single scale and single index of the attenuator gives a correct invice versa. In the arrangement shown in Fig. 1,

brake pad |3| being pressed, as by a spring, against the support of the attenuation scale AS which forms part of the attenuation regulator.

'Ihe motion of the attenuator contact slider |02 with is attenuation scale AS relatively to the attenuator support |24 is limited by two stops |34, extending from the attenuator support |24 adjacent the ends of the main resistor pad IUI.

The stops |34 are positioned and arranged relatively to the slider contact |02 and the resistor pad |0| as to prevent inaccurate adjustment of the attenuator and to assure that the attenuation settings are always accurately indithe receiver is able to deliver at any given frequency setting, the slider contact |02 of the attenuator being automatically engaged by one or the other of the attenuator stops |30,A |31 and prevented from being brought to a position of incorrect attenuation setting or incorrect attenuation indication.

If an audiometer arranged in the way described above is used for making measurements with the air conduction receiver AR, with the attenuator set for a hearing loss of 20 decibels, indicated by its index AI at 20 on its scale ASA, and the frequency selector is swept at the same hearing sen'- sation level of 20 decibels above the threshold of hearing, -as indicated in Fig. 2 by the curve marked 20, the attenuation support will be automatically shifted to attenuate the oscillatory current delivered to the receiver to a greater degree in the range below 400 cycles and above 3000 cycles, to compensate for the varying sensitivity of the receiver and the other factors which vary' with the frequency, for instance, in the way indi, cated bythe curved parts of the cam track |2i in the parts of the frequency range below 500 cycles and above 2500 cycles, and the intermediate substantially even part of the cam track bedication of the hearing loss and that on sweeping Provision is also made to prevent disturbance of the attenuation indication if the frequency selector is swept through the frequency range while the attenuation loss setting is higher than that corresponding to the sound whichthe receiver is able to deliver in parts of the swept i'rc-A quency range.

In the form of the audiometer shown in Fig. 1,.the attenuation by index AI, indicated on the attenuation scale AS, may be adjusted by shifting the slider contact |02, which serves as the attenuation regulator, along the resistor pad, a

tween about 500 and 2500 cycles. Let it also be assumed that the dash line curve ML in Fig. 2 represents the maximum-sensation level which the receiver AR. is able to deliver at the different frequencies when energized by the maximum available oscillator power. c

Ina similar way, if the attenuator is set for zero sensation level, as indicated at zero on its attenuation scale ASA, and the frequency sev lector 9| is actuated to sweep throughout the` frequency range, it will move the attenuationsupport so that at each frequency, the electric output delivered to the receiver through the attenuator network will produce in the receiver a sound intensity indicated by the lower curve threshold of audibilityv of Fig. 2, this beingthe sensation level zero on the attenuation scale ASA.

lOn the other hand, if the attenuation is set for a sensation level and the frequency selector 9| is swept,`for instance, from the frequency setting of 2000 cycles down toward the lower end of the frequency range, the cam track |2| will shift the attenuator support so as to deliver the receiver oscillatory currents which produce a sound intensity corresponding to a hearing sensation level of 80 db., following curve 00 in Fig. 2 untilits crossing point ML 80 with curve ML is reached, at which the slider contact |02 of the 'attenuator engages one of its stops |34, so that in the course of the further sweeping motion of the frequency selector 8| its cam track |2| drags the attenuator Y support |24 with its resistor pad |0| as well as the contact slider |02 with its attenuation scale, against the braking action of its braking pad |3I, so as to indicate on the 'attenuation scale` ASA the correct reduced sensation level of hearing along curve ML, from' point ML80 down to the lower end of the frequency range, corresponding to the smaller sound intensity which can be produced and measured at such lower part of the frequency range.

` Similarly,/if the attenuation level is raised, by moving the attenuation contact slider |02 with its scale AS, while the/ frequency selector 9| is in a.

setting such as 128 cycles, one of the attenuator stops |34 will come into play to prevent an incorrect adjustment of the attenuator, to a setting higher than that corresponding to the output which the receiver isV able to deliver at the given frequency setting. In other words, as long as the frequency selector `or attenuation regulator is adjusted to settings, at which the test receiver is the maximum output which the receiver is able to deliver in such parts of the frequency range. In the audiometer arrangement of the invention, the frequency selector is thus so combined with the attenuator, and the cam tracks which control the operation of the attenuator in accordance with the operation ofdthe` frequency selector are so arranged andl shaped that for each frequency setting, the attenuator automatically'brings the slider contact into engagement with ax portion ofits rheostat network to secure controlled attenuation of the oscillatory output supplied to 'the attenuator network required in order to produce in the receiver the sound `output which gives the sensation level indicated on the attenuator scale, and thus assure compensation for the relative variations in the sensitivity ofthe receiver and of the oscillatory able to deliver the sound intensity corresponding to the indicated settings, changing the frequency settings throughout the frequency range will not disturb the attenuator setting, and changing the attenuator setting will not disturb the frequency setting. However, if a change in a frequency setting into a part of the frequency range in which the receiver is unable to produce the sensation level indicated on the attenuation Iscale, one of the stops |34 of the attenuation will come into play and prevent the frequency selector from being moved into the marginal frequency scale regions-without at the same time dragging the attenuation support to a position correspondlng to the lower sensation level available at such frequency settings. I

This arrangement makes it possible to regulate the attenuation so that the sound outputV delivered by the receiver is at the same sensation level throughout the frequency range available for each distinct setting of the attenuation regulator. As th'e sensation level is increased above that available` at the end frequencies, namely, the low frequency and the high Afrequency range, the stop of the attenuatoroomes into play to prevent the frequency selector from moving into the marginal frequency scale regions without dragging the attenuator body to a position corresponding to the lower sensation level available in such region as indicated on the attenuation scale.

In other words, the frequenc'y selector means and the attenuation regulating means are so correlated that the sound intensity delivered by the receiver is always at the sensation level indicated on the attenuation scale as long as the generated frequency is varied throughout the frequency range available for each setting of the attenuator scale, while assuring that if the Sensation level, as indicated on the attenuator scale, is increased above that which the receiver is able to deliver in the lower and higberfrequency ranges, the attenuator regulating means are automatically actuated to prevent the frequency selector from moving into these 'regions without at the 'same time adjusting rthe attenuator to a lower sensation level setting corresponding to output delivered by the oscillation generator as well as any other variable factors at each particular frequency. The cam track which is operated by the frequency selector is so shaped and arranged that when the attenuator is moved to a setting corresponding to the maximum sound output that the receiver is able to deliver at the given frequency setting, one..,of the attenuator stops comes into piayand prevents actuation.w

of the attenuator to an incorrect setting Thus, if the frequency selector is moved to the 125 cycle setting,A its cam track actuates the attenuator support to a position where its stop makes it impossible to adjust the attenuator to an attenuation setting corresponding to a sound output higher than that corresponding to the maximum output which the receiver is able to Y deliver at such low frequency. Similarly, for each other frequency setting, the stop of the attenuator support prevents the attenuator from being adjusted to a position, corresponding to an attenuation setting higher than the receiver is able 'to deliver at the given frequency' setting. In order to maintain the attenuator setting as indicated on its scale while the attenuator support is actuated by the frequency selector, the

attenuation regulator is provided withfa brake .arranged so that the motion imparted to the Y attenuator support does not normally affect the lcial form of audiometer embodying the principles of the invention described in connection attenuator setting as indicated on the attenua ator scales.

The principal structural features of a commerwith Fig. 1 will now be described. As shown in Figs. 3 to 6,*a frequency selector'or control shaft |36,y carrying at its inner end the frequency selector plate 9|, is rotatably mounted in a mounting wall |31, located behind a front panel c |38 of the audiometer, and has secured to its frontend a frequency control operating knob FC. A dial with the frequency scale FS is affixed to the frequency selector shaft |36 behind a window ESW of the front panel |38 through which a portion of the frequency scale is exposed, and

a strip |39 of transparent material bearing the frequency index FI being placed immediately behind the windowv for indicating on the scale FS the different frequency settings.

The rotary condensers Cl, C2 of the feed back networks LF, HF are mounted together with the other elements of the feed back networks in a shielded casing |40 of metal supported within a suitable audiometer housing, in the space extending behind the mounting wall |31, the shaft 94A of the rotary condensers of the low frequency network LF carrying on its outer end the gear feed back network HF carrying on its outer end the condenser gear 95.

Cheap commercially available rotary gang condensers and cheap gears 93, 95 are used. The gears 93, 95 are driven by the frequency selector member 9|, shown as a circular disc made of a similar gear blank from which about halfy of the gears have been removed so as t1, provide thereon a gear segment 92 extending along about half of its periphery for driving either one vof the condenser gears 93, 95 of thetwo feed back networks in a way analogous to that described in connection withFig. 1.

In the form shown, the driving gear disc 9| and the twocondenser gears 93, 95 are made of standard gear blanks having the same wall thickness. 95 are slightly offset relatively tov the teeth of the driving gear segment 92 of the frequency selector disc 9|, and the portion of its periphery which has no teeth has `secured thereto a locking plate segment |4| arranged so that its perheral edge is aligned in overlapping relation to the lteeth of the two condenser gears 93, 9l5 and serves as the locking cam sections 96, 91 for interlockingly engaging one of the twocondenser gears 93, 95 while thev other is rotated by thev driving lgear 92, except for the duration of the short transition period during which the driving gear segment 92 engages and drives both condenser gears 93, 95 in a way analogous to that explained in connection with Fig. 1.

The side wall portions of the condenser gears 93, 95 which are aligned in the plane of the locking cam sections 98, 91 of the locking segment |4| are provided with shallow depressions |42 arranged so that a tooth portion bordering one peripheral edge of the depression serves as the locking projection 98 which becomes engaged by the periphery of the locking segment |4| when one ofthe condenser gears 93, 95 is locked.

In the form shown in Figs. 4 to 6, the clockwise rotation of the frequency selector disc 9| "imparts through its driving gear segment 92 a counterclockwise motion to the condenser gear 93 of the condenser rotor plates of the feed back network LF, rotating them counterclockwise from their inward to their outward position, While the peripheral edge of the locking plate |4| engages the overlapping tooth projection 98 of the gear 95 The teeth of the condenser gears 93,v

l t .of the gear segment 95 so that during the short transition period, until the condenser rotor plates dri-,ven by gear 93 reach the end of their outward path, the rotary condenser platesl of both feed back networks will be rotated by the driving gear 92 of the frequency selector disc 9|.

At the end of the transition period, in the course of the rotation of the frequency selector disc 9|, its peripheral locking segment portion 98 comes into engagement with the locking teeth projectionl 98 of the condenser gear 93, and locks it and the rotary condenser plates of the low frequency network LF against counterclockwise rotation, a stop projection |43 mounted in the path of a stop pin |44 extending, for instance, from the hub of the lcondenser gear 93 preventing counterclockwise rotation thereof.V As a result,

during the further clockwise rotation of the fre-` quency selector disc 9|, the condenser rotor plates of the low frequency feed back network LF remain locked in their outward position, whilel the frequency selector gear 92 rotates through and holds it locked so that the condenser rotor plates of the high frequency feed back network- HF are kept locked in theirinward position in which the condensers have maximum capacity.l

condensers are usually designed so that when their rotary plates are in the inward position, they can be' rotated only in outward direction, the rotary condenser plates of the feed back network HF will remain'locked in their inward position as long as the locking projection 98 of the condenser gear 95 is engaged by the locking segment |l4| of f' the frequency selector disc 9|' and is prevented fromturning in outward counterclockwise direction. f i

`Shortlybefore the counterclockwise rotation of condenser gear 9 3v brings its rotary condenser platesto the end of their outward Path, the lockingf'projection 98 of the gear 95 isjreleased from itslfdcking engagement with the peripheral locking 'segment'portion 91 of the frequency selector 9| and 'the'ajdjoining teeth of thedriving gear *segment 92 'come into engagement with the teeth condenser gear 95 the rotary condenser plates of the high frequency network from their inward toward their outward position.

In an analogous Way, counterclockwise rotation of the frequency selector 9| will return first the condenser rotor plates of the high frequency network HF that are connected to the condenser gear 95` from their outward position to their i-nward position and after locking them in their inward position, at the end of the transition period, the further counterclockwise rotation of the frequency selector disc 9| returns the rotary condenser plates of the low frequency network from their outward to their inward position.

"As shown in Fig. 6, the condenser gear 93 of with the insulating insrt 99 for controlling the operation of a spring contact switch 33 whichv operates the relays in the way described in connection with Fig. l.

The rotational movement of the driving gear 92 is limited by a stop pin |46 extending from its body and engaging in its endpositions either the right or the left side of a stop |41 suitably mounted in the path of the stop pin`|46.

As shown in Figs. 3,y 4 and 8 to 10, an attenuatorcontrol or regulating shaft |52 carrying on an insulating support |53 secured to its inner end, the contact slider |92 of -the attenuator is rotatably mounted in the mounting wall |31 of the audiometer and has secured to its outer end projecting through the front panel |38 an attenuator control knob AT similar to the frequency selector'knob FC. A dial plate provided,-

with the attenuation scales AS is secured to the attenuator control shaft |52 so as to expose through a window ASW in the front panel |38 a portion of the attenuation scale AS, a plate of transparent material |53 carrying an attenuation index marked AI in front of the exposed portion of the attenuation scale AS so as to indicate the setting of the attenuator.

As shown' in Figs. 8 and 9, a circular metallic housing which forms the attenuator support |24 is revolvably mounted on attenuator control shaft |52 which carries the slider contact |02,

l the front side of the attenuator support housing |24 being provided with a collar |53 on which is mounted a gear |54 by means of which the attenuator Yhousing |24 withits attenuator support |24 may be independently turned relatively to the control shaft |52 andthe slider contact |02 carried'thereon.

n a suitable cup shaped insulating support 4 |56 affixed to the interior of the attenuator housing |25 are mounted two circularly bent wirewound resistor pads |0|, |03 so that when the attenuator housing |24 is rotated relatively to this attenuatorcontrol shaft |52, the slider coni thereon, and similar jumpers interconnect taps on the exterior resistor pad |03 to the bus conductor |04 strung over an insulating collar placed around the outer resistorpad |03.y The stop |34 of the attenuator supporting housing is formed,

by a projection from the insulating cup |55 and is located in the path of a stop projection |02-S extending from the slider contact support |53 so that when in the course of the rotation of the attenuator housing, its stop |34 engages the stop |02-S of the slider contact |02, the further rotary motion imparted to theattenuator housing |24 will also drag with it the sliding contact |02 as well as its control shaft |52 and the attenuation scale AS. An additional support for the attenuator housing |24 while it revolves relatively to the attenuator control shaft |52 is provided by a bracket |56 suitably secured to the mount ing wall |31 and having at its center a journaling hole serving as a journaling support for a pin |51 extending from the rear of the attenuator casing |24. f

As shown in Fig. 4, the operation of the frequency selector shaft 9| and the operation of the attenuator with its attenuator support |24 are correlated, by alfixing to the frequency selector shaft 9| a circular cam plate |20 having twoy cam tracks |2|, |22 for air conduction and bone conduction measurements, respectively, arranged to be engaged with either one` or the other of4 ment of the attenuator housing |24 with its resistor pad lill relatively to the contactslider m2 in a way analogous to' that described in connection with the diagrammatic showing of the corresponding parts in Fig. 1.

ing from one of the cam follower brackets |64 so that as long as one of the guide pins |25', |26 is J confined within the guide channel |66, the cam rod |21 cannot beturned from one operating position to the other.

The guide channel |66, in which one or the` other of the guide pins |25', |26 of the cam rod |21 is confined during its longitudinal movement,

is made long enough so as to release the confined guide pin when the cam rod |21 is inthe position of its maximum displacement in thedirection toward the frequency selector shaftQI correspond- As shown in Figs. 4 and 10, the cam follower rod |21 has a circular cross-section and is guided longitudinally by supporting brackets |64,V suitably secured t0' the rear of the mounting panel |40. The cam follower rod |21 may be turned by 96 within its guide brackets |64 so as to bring its cam follower pins |25, |26 into engagement with either one of the cam tracks |2|, track |22, respectively, depending on whether hearing by air conduction or bone conduction is to be measured. l

Provision is also made to assure that only in one position ofthe frequency selector 9| is it possible for the audiometer to be changed from air conductionv testing to bone conduction testing. To this end, one of two guide pins |25', |26', projecting from the cam follower rod |21 in alignment with its corresponding cam follower pins ing to a setting of the frequency selector at which the change to other type of testing should be made.

In the form of arrangement shown, the changeover from measuring hearing by air conduction to bone conduction and vice versa can be made onlywhen the frequency selector 9| is in lthe position of its lowest frequency settings in which position the confined guide pin of the cam follower rod |21 is released from engagement with the guide channel member |66, and the cam follower rod |21 may be turned 90 for changing to the other type of measuring hearing.

The limited changeover motion from air conduction to bone conduction testing or vice versa -is imparted to the cam follower rod |21 by an operating lever |35 which is secured to agear member |1| mounted on the shaft |12 of a switching unit il@ suitably secured as by a bracket to the. rear side of the mounting wall |31 of the attenuator, the teeth of the gear |1| meshing with an elongated pinion |14 slidingly keyed to the cam follower rod |21 so as to maintain engagement with the gear |1| in all itslongitudinal positions.

The switching unit ||0 has the various contacts shown diagrammatically in Fig. l, which are operated when the lever |35 is moved to the air conductionor bone conduction positions, its gear |1| imparting at the same time through the pinion Ht a turning motion to the cam follower rod |21 for releasing one and engaging the other of its cam pins |25, |26 with the corresponding cam track of cam plate |26.

In order to assure that only one of the two spring i12 so as tobe movable between two end positions for exposing through one of its windows a portion of the air conduction scale ASA or a portion of the bone conduction scale ASB.

The 'changeover lever |35 is suitably coupled to the masking plate |1I, for instance, by engaging a hole formed in it, so that when the changeover switch |0 and the cam follower rod |21 are moved from the bone conduction position to the air conduction position or vice versa, the masking plate |1| is automatically tilted to the position in which it exposes behind the attenuation index AI either a portion of the air conduction scale only or a portion of the bone conduction scaleonly.

It will be noted that, as shown in Figs. 1 and 4, the cam tracks |2|, |22 which are utilized for correlating the operation of the frequency selector and the attenuation regulator are of irregular character. This is due to the fact that the factors. whlchrequire correlation vary with the frequency in a very irregular manner and do not conform to any regular law or function.

In Fig. is shown a modified arrangement of audiometer exemplifying in another form some of the principlesof the invention explained in connection with Fig. 1. l

It comprises an oscillation generator tube 2 I2; two push-pull connected amplifier tubes 2|6; and a network of inductances, condensers and asso-,- ciated elements for enabling selective generation of the required pure tone frequencies. The tubes are energized from a conventional 11o-volts electric supply line 220 throughl a power pack 22| which is connected to' the supply line by a power switch 222. The power pack 22| comprises a rectifier 223, a filter choke 224, and two lter condensers 225 connected in shunt to the grounded lead of the filter. A meter 226 and rheostat 2'21 enables accurate adjustment of the direct current voltage delivered `at the positive filter terminal +B, to assure proper operation of the oscillation generator. The four heaters of the cathode 2|| are shown energized through a paralleisupply circuit including a rheostat 228, a signal light 229 and a pilot light 230.

The plate 2|4 of the oscillatortube 2|2 is connected through a choke coil 23| of the B+ terminal of the power pack, and its cathode 2|3 is connected in series with a self-biasing resistor 232 to the ground, a by-pass condenser 233 beingv connected parallel to the resistor 232. The plates of the amplifier tubes 2|6 are connected in pushpull to the primary winding of an output transformer 235, the mid-point of which-is connected through a lead 23B to the plate supply terminal +B, and the cathodes of the amplifier tubes 2|6 are connected through a resistor 231 to ground.

'I'he oscillation generator is designed to operate with fixed tuned circuits for producing pure tone oscillations of the different frequencies important in making audiometric tests, namely, the frequenciesv separated by an octave in the range between 128l t`o 8192 cycles per second. To this end, the oscillator tube 2|0 is arranged to 7 selectively cooperate with two tuning inductances Ll, L2, a bank of tuning condensers C| to Cl, and a bank of feed-back resistors RI to Rl, as vertically aligned with the corresponding tuning condensers with which they are selectively interconnected to the oscillator tube for producing the oscillations of the different frequencies. A bank of multi-blade contact-spring switches, designated by the numerals 128 to 8192 corresponding to the different frequencies, each switch having a set of contact blades SO, SA, shown as'arrows, vertically aligned with the corresponding tuning condensers C| to C1, and feed-back resistors Rl to R1, serve to interconnect the selected combination of tuning nductance, 'tuning condenser, feed-back resistorsy into the tuned oscillating circuits required to generate the different frequencies. To complete an oscillating circuit for a given frequenccy, for instance, for 1024 cycles. the end blade E of the switch |024 is flexed to actuate its other aligned contact blades to the closed position, thereby establishing the following operating circuits:

From the plate |4 of the oscillator tube by wayv of lead 24| including blocking condenser 242 to `the feed-back resistor R4, by way-of the aligned closed blade contacts of selector switch |024 to the tuned circuit formed of the condenser C4 and the inductance L2 which is connected by leads 24,3', 244 to the condenser C4, continuing ,from the the closed contact blades and lead 245 to the control grid 2|5 of the oscillator tube 2|2, the grounded mid-point of the inductance L2 completing the plate circuit of the oscillator tube 2 |2 and the feed-back circuit to the control grid 2|! of the oscillator tube 2|2, the feed-back resistor R4 being adjusted to securel pure sinusoidal oscillations of 1024 cycles fixed by the tuned circuit. In addition, the closed contacts of the selector switch |024 connect the opposite sides of the tuned circuits C4, L2 by way of leads 246, 248 to the control grids 2|5 of the two push-pull connected amplifier tubes 2|6, so as to deliver' amplified 1024 cycle oscillations to the .output transformer 235. 4

In a similar way, oscillations of each of the other testing frequencies may be selectively generated and delivered to the output transformer 235 by actuating, the end spring E of any one of the other selector switches |28, 256 8|92 from the normally open position to the closed po- Isition in the'way described above in connection with the oscillating circuit for the 1024 frequency. The oscillator arrangement 'described above is distinguished 4by a number of important charent pure tone oscillations in the frequency range' between 128 cycles and 8192-cycles. Each inductance must thus serve with a series of different condensers as a tuned tank circuit of resonant frequencies, differing from each other by three octaves. As a result, the impedance of the tank circuit, formed by the inductance in `combination with the different condensers greatly increases as the frequency increases. Such large tank impedances require, in turn, very large feed-back resistors to prevent distortion of the oscillations, and such large feed-back resistors must be accurately adjusted so as to be free from spurious effects that would interfere with the proper operation of the oscillation circuits. In the oscillator arrangement of the invention, these difficulties are overcome, by providing for-each other side of the tuned circuit C4, L2, by way of tuned tank circuit a shunting resistance, formed of the two resistors 249 arranged to be connected in parallel to. the tuned circuits of the different frequencies so as to limit the maximum impedance across the tank circuit to a value of about 100,000 ohms. This, in turn, makes it possible to use moderate size feed-back resistors Rl to R1, assuring stable generation of the oscillations of the different frequencies.

By using'the different tuned tank circuits required to generate the oscillations of the different frequencies as a coupling impedance to the push-pull amplifier, the tank acts as a very effective lter for harmonics that might produce distortion. By this arrangement, the oscillatory Y voltage which is impressed by the oscillator on the control grids of the push-pull amplifier has a very pure wave formand the push-pull ampli-v fier is able to deliver oscillations of great purity.

In order to actuate the oscillation generator to selectively generate the desired frequencies, the frequency lselector 2.|0 is made in the form of a movable member having, for instance, a cam nose 266 'which may be movedinto selective engagement with the end blade E of'any one of the selector switches so as to iiex its associated contact blades from the normally open position to the closed position, in which the oscillator cir-- cuits of the selected frequency are established so as to generate and deliver to the output transformer 236 the desired frequency, as explained above for the 1024 frequency setting.

The oscillations of the different frequencies are delivered to an attenuator 260 connected to the secondary winding of the output transformer A3l. 'I'he attenuator comprises a plurality of sedicate by an index 261 on the scale 266 the hearing loss in decibels corresponding to the output delivered by the attenuator network at the tap 264 engaged by the sliding contact 265, the attenuation regulator 2|I being movable in the direction of the arrows, and back, to bring the sliding contact into contact engagement with any one of the taps 264 of the attenuator network. i

An air conduction receiver 218 and a bone conduction receiver 21| are arranged to be connected between the attenuator lead 263 and a switch 212 which is connected through lead 213 to sliding contact 265 to enable energization of either one of the receivers with the attenuatedl oscillatory output toproduce sound energy of the frequency vdetermined by the frequency selector 2|0,.and of an intensity level determined by the attenuation regulator 2| There is also provided an arrangement, which correlates the operation` of the frequency selector with the attenuation regulator and the control of the .intensity of the sound delivered by the receiver to assure proper operation of the audiometer. It comprises a plurality of shunting resistances Ai to A1 and Bi to B1 interconnected to a lead 215 that is connected to the lead 263 from one end of the secondary winding of the output transformer 235. The other ends of the resistors Al to A1 and Bl to B1 are arranged to be selectively connected by the Contact blades SA.

of the selector switches |28 M82 to leads 216, 211 which, in turn, are arranged to be connected by switch 218 to the lead 219 extending from the other end of the secondary winding of the output transformer 285. The switch '21d is operated conjointly with switch 2W which controls the connections of the air conduction receiver or bone conduction receiver 21| to the attenuator so that lead 216', which `completes the connection to the resistors vAI to A1, is connected to the output transformer 235 when the air conduction receiver `210 is energized, and lead 211, which completes the connection to the resistors B l to B1, is connected to the output transformer 235 4when the bone conduction receiver 21| is energized. Y q

With this arrangement, each setting of the frequency selector automatically connectsone of the equalizing resistors AI to A1 or one of the equalizing resistors BI to B1 in shunt to the secondary winding of the output transformer 225 while the bone conduction or air conduction receiver, re-

spectively, is energized by the attenuator. The individual resistors AI to A1 are adjusted to make the electric output delivered by the output transformer 235 to the attenuator such that for each of the different testing frequencies, the attenuator scale gives an accurate indication of Athe sound intensity delivered by the air conduction receiver in terms of the hearing loss. Similarly, the individual resistors BI to B1 are adjusted to equalize the electric output delivered by output transformer 235 to the attenuator so that for each of thev different testing frequencies, the attenuatorscale gives an accurate indication of the sound intensity delivered by the bone conduction receiver.

'I'he function "of the equalizing resistors AI to A1 and Bl to B1 is to reduce the intensity of 4the oscillations delivered to the input side of the attenuator and thus compensate for the differ-- ences of the sensitivity of the receiver and'alsol for variations in the characteristics of the oscillator at the different frequencies of the audiometer range. Obviously, such resistance, or, in general, any impedance-type equalizing network effects equalization of electrical output energy only by reducing the maximum output In other words, the electrical output delivered by the oscillation generator must be at least as large as the maximum output required by the receiver under its most unfavorable operating conditions.

To assure that for each setting of the attenuator, the intensity of the sound delivered by the receiver is accurately indicated on the attenuator while the setting of the frequency selector is changed over the frequency range, the operation of the frequency selector is automatically correlated with the attenuation scale arrangement to assure that if the frequency selector is shifted, from a position in the mld-frequency range between about 400 to 2500cycles in which the receiver is able to deliver a db. output to a position in marginal frequency ranges in which the receiver is unable to deliver the maximum output of the-100 db.sound intensity, the attenuation index on the attenuator scale is automatically brought to a position which makes it impossible to make on the attenuation scale a reading indicating a sound intensity greater than that which the receiver is able to deliver.

The frequency selector 2 |0 has a cam member 280 arranged yto cooperate with a cam follower 28| which holds the attenuation scale index 281. The cam follower 28| is shown guided in sleeves 282 and is urged by a biasing spring `283 against the cam surfaceof the cam member 288 so as to slide thereon and be brought into engagement with different portions of the cam surface while the frequency selector is moved to the dierent positions indicated by the frequency index 252 on the frequency scale 25|.

y In the form shown, the cam surface of the cam 286 1s so arranged that as long as the frequency setting of the frequency selector remains within the mid-frequency range, the index 2 51 is retained by the cam follower 28| in a fixed position so as to indicate on the audiometer the maximum 4 extent of the attenuation required to determine quency selector is set to generate a frequency within the range at which the receiver is able to deliver 100 db. above the threshold of hearing, for instance, between Vaboutl500 and 2000 cycles,

the cam follower 28| with its attenuator index 281 will remainflxed in the position at which the attenuator scale is able to indicate the maximum hearing loss shownon its scale, in the case illustrated, 100 db.; and that as soon as the frequency selector 2 I0 is moved to a position where its index indicates on the frequency scale :a frequency at which the receiver is not able to deliver sound intensity corresponding to a 100 db. hearing loss, such as at a frequency between 200 to 888 and between 3000 to 5000 cycles, the cam follower 28| is brought into engagement with the cam surface portions 288, 281, respectively, thereby shifting the attenuation index to a position in which the attenuator scale can indicate only the maximum output that the receiver is able to actually deliver at such marginal frequencies, in the case illustrated, 90 db. hearing loss on the attenuation scale 288; and that on moving the selector to a marginal frequency range below- 200 cycles or above 5000 cycles at which the receiver is unable to deliver 90 db the follower 28| is brought into engagement with the cam surface portions 288,' 289 so as to shift the attenuation index 281 to a position in which it makes it impossible for the attenuator to be regulated to indicatel a hearing loss corresponding to a sound intensity greater than the receiver is able to deliver in the frequency range corresponding to the vsetting of the frequency selector, in the case illustrated, 75 db. hearing loss.

'I'he several cam surface elements 285 to 288 are so arranged that, as the frequency selector is moved to the different frequency settings indicated on the frequency dial, the cam follower 28| which fixs the position of the attenuation index, or, in general, controls the attenuation scale arrangement, automatically moves from its engagement with one cam surface portion into engagement with another -camsurface portion corresponding to the different frequency range settings of the frequency selector. Obviously, depending on the sensitivity of the receiver in the different frequency ranges and depending on the maximum output that the receiver is able to deliver at the different frequency ranges, the dif` ferent cam surface portions may be arranged to permit the operation of the attenuator to indicate the maximum output that the receiver can deliver at the given frequency.

It is likewise obvious that if the audiometer of the type described above were required to explnre a maximum loss of hearing of 75 db. above the normal threshold of hearing throughout the cretire frequency range between 100 and 8000 cycles, the cam surface of the cam member 280 Theinterrupterarrangement comprises an inte'rrupterV switch 29|, which is normally in its back contact position and a resistor 282 connected in series with the lead whichapplies the operating bias to the screen grids 2|1" of the amplifier tubes 2|8 so as to maintain them at their normal operatingy potential. When the interrupter switch 29| is actuated from the position shown to the front contact position, it opens the connection of the screen grids 2|1 to the positive bias terminalV +B and connects the screen grids to the ground and to a shunting condenser 293 which in the normally closed posi# tion of switch 29| ismaintained fully charged to the normal screen grid potential. As a result, actuation of the interrupter switch 29|, to disconnect the screen grids 2 I5 from the +B poten tial, does not instantaneously apply the ground potential to the screen grids, but the condenser 298 which is fully charged to the screen grid potential only gradually discharges across the resistance 292, thereby gradually reducing the bias of the screen grids 2|5 so that the tone dies away slowly without causing any clicks or other masking noises that would disturb the individual under test and affect the results of his tests. Restoration of the tone, by returning the interrupter switch 29| to the back contact position shown, does not suddenly` apply the full screen potential to the screen grids 2|1, because during the initial period while the condenser 298 is being recharged, they potential applied to the 'screen gradually rises and the level of the tone gradually rises without producing any disturbing masking noises or clicks.

The switch controlled signal light 229 makes it possible for the person under test to indicate the moment when he ceases to hear the sound. The energizing circuit 'of lamp l229 is provided with a signal switch which is manually operated by the person under test who is usually instructed to release and press the lswitch when he hears and ceases to hear, respectively.

In order to make it possible to converse with a deafened person who is undergoing a test, the

push-pull amplifier associated Iwith the tubes 2|8 for amplifying the test oscillations may be also utilized to amplify speech directed to the person ranged to `connect the microphone through' the input transformer 298 so as to impress the con- Y trol grids` 2|5 of the two amplifier tubes 2| 5 the would be designed to keep the index 281 at all times in a fixed position where the maximum vided with a tone interrupter arrangement whichenables momentary interruption of a tone without producing a click might mislead theindividual who is being tested.

speech current transmitted by the transmitter 294 when the switch 295 is moved from the normal position shown in Fig. 3 to the position in which .the microphone 298 is connected to the secondary Winding of the transformer 296 and the control grids of the amplifier tubes 2|6 are disconnected from their normal operating circuits and connected tothe secondary winding of the transformer 296. The primary winding of the transformer 296 is shown connected by means of a network including a choke coil 281 'and a condenser 298 connected across the resistor 231 so as to supply to the transmitter mi' or any other noise that crophone 294 a D. C. exciting current.

'I'he distinct novel features of the oscillator arrangements and attenuator arrangements disclosed above in connection with the description of measuring and testing arrangements exemplifying the features of invention claimed herein, do not fo rm the subject matter of the present application and are claimed in my copending application Serial No. 395,927, tiled May 31, 1941, 

